An atomic force microscope (AFM) is a comparatively high-resolution type of scanning probe microscope. With demonstrated resolution of fractions of a nanometer, AFMs promise resolution more than 1000 times greater than the optical diffraction limit.
Conventional AFMs include a microscale cantilever with a sharp tip (probe tip) at its end that is used to scan the surface of a specimen or sample. The cantilever is typically silicon or silicon nitride with a tip radius of curvature on the order of nanometers. When the probe tip is brought into contact with the sample surface, forces between the probe tip and the sample surface lead to a deflection of the cantilever. Typically, the deflection of the cantilevered probe tip is measured using a laser spot reflected from the top of the cantilever and onto an optical detector. Other methods that are used include optical interferometry and piezoresistive AFM cantilever sensing.
Another component of an AFM is an actuator, which maintains the angular deflection of the tip that scans the surface of the sample in contact-mode. Most AFM instruments use three orthonormal axes to image the sample. The first two axes (e.g., X and Y axes or a horizontal plane) are driven to raster-scan the surface area of the sample with respect to the probe tip with typical ranges of 100 μm in each direction. The third axis (e.g., Z axis or vertical direction) drives the probe tip orthogonally to the plane defined by the X and Y axes for tracking the topography of the surface.
Generally, the actuator for Z axis motion of the probe tip to maintain a near-constant deflection in contact-mode requires a comparatively smaller range of motion (e.g., approximately 1 μm (or less) to approximately 10 μm). However, as the requirement of scan speeds of AFMs increases, the actuator for Z axis motion must respond comparatively quickly to variations in the surface topography. In a contact-mode AFM, for example, a feedback loop is provided to maintain the probe tip in contact with a surface. The probe tip-sample interaction is regulated by the Z feedback loop, and the bandwidth of the Z feedback loop dictates how fast scanning can occur with the Z feedback loop remaining stable.
Some conventional AFM systems also provide optical viewing access to the sample surface in addition to the AFM imaging. The user of the AFM may employ the optical viewing access to control and expedite certain operations of the AFM. However, the user must perform tedious manual adjustment, and thus act as a feedback loop, to correct inaccuracies or variations between the AFM and optical viewing access. For example, when the user wants to perform AFM experiments at a certain position related to a feature of interest, inaccuracies of motorized stage movements must be corrected manually based on the optical image. Such inaccuracies and subsequent manual corrections inhibit automated measurements and slow the analysis process. Further, in conventional AFM systems, the user manually moves the probe tip to within at least a millimeter above the sample surface, and then very slowly moves the tip toward the sample surface to avoid of crashing the ultra-sharp probe tip into the sample. The time required for this manual approach process typically takes about 5 to 15 minutes.
There is a need therefore, for an AFM apparatus that overcomes at least the shortcoming of known AFM apparatuses described above.